• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.447-452
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• 2001

Quenching phenomena of hemispherical downward facing convex surfaces with narrow gaps have been investigated experimentally. Experiments employed test sections having 1 and 2 mm in gap thickness and 1 atm in system pressure. From interpretations of the temperature and the heat flux history, it was found that the flooding inside the gap was restricted by CCFL phenomena and quenching process was propagated from lower to upper region of the internal copper shell. The ratio of the maximum heat fluxes at 1 mm to 2mm in gap thickness was the almost same that obtained by steady state experiments. The quenching scenario of the hemispherical downward facing surface with narrow gap has been suggested.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2013년도 춘계학술대회 논문집
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• pp.835-841
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• 2013

Modern solid-state gyroscopes (HRG) with hemispherical resonators from high-purity quartz glass and special surface superfinishing and ultrathin gold coating become the best instruments for precise-grade inertial reference units (IRU) targeting long-term space missions. Designing of these sensors could be a notable contribution into development of Korea as a space nation. In participial, 40mm diameter thin-shell resonator from high-purity fused quartz, fabricated as a single-piece with its supporting stem has been designed, machined, etched, tuned, tested, and delivered by STM Co. (ATS of Ukraine) several years ago; an extremely-high Q-factor (upto 10~20 millions) has been shown. Understanding of the best way how to match such a unique sensor with inner glass assembly of the gyro means how to use the high potential in a maximal extent; and this has become the urgent task. Inner quartz glass assembly has a very thin indium (In) layer soldered the resonator and its silica base (case), but effects of internal resonances between operational modal pair of the shell-cup and its side (parasitic) modes can notable degrade the potential of the sensor as a whole, instead of so low level of resonator's intrinsic losses. Unfortunately, there are special combinations of dimensions of the parts (so-called, "resonant sizes"), when intensive losses of energy occurs. The authors proposed to use the length of stem's fixture as an additional design parameter to avoid such cases. So-called, a cyclic scheme of finite element method (FEM) and ANSYS software were employed to estimate different combinations of gyro assembly parameters. This variant has no mismatches of numerical origin due to FEM's discrete mesh. The optimum length and dangerous "resonant lengths" have been found. The special attention has been paid to analyses of 3D effects in a cup-stem transient zone, including determination of a difference between the positions of geometrical Pole of the resonant hemisphere and of its "dynamical Pole", i.e., its real zone of oscillation node. Boundary effects between the shell (cup) and 3D short "beams" (inner and outer stems) have been ranged. The results of the numerical experiments have been compared with the classic model of a quasi-hemispherical shell band with inextensional midsurface, and the solution using Rayleigh's functions of the $1^{st}$ and $2^{nd}$ kinds. To guarantee the truth of the recommended sizes to a designer of the real device, the analytical and FEM results have been compared with experimental data for a party of real resonators. The consistency of the results obtained by different means has been shown with errors less than 5%. The results notably differ from the data published earlier by different researchers.

• Structural Engineering and Mechanics
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• 제68권1호
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• pp.17-38
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• 2018

In this paper, we compare and assess the performance of the standard 3- and 6-node MITC shell elements (Lee and Bathe 2004) with the recently developed MITC triangular elements (Lee et al. 2014, Jeon et al. 2014, Jun et al. 2018) which were based on the partitions of unity approximation, bubble node, or both. The convergence behavior of the shell elements are measured in well-known benchmark tests; four plane stress tests (mesh distortion test, cantilever beam, Cook's skew beam, and MacNeal beam), two plate tests (Morley's skew plate and circular plate), and six shell tests (curved beam, twisted beam, pinched cylinder, hemispherical shells with or without hole, and Scordelis-Lo roof). To precisely compare and evaluate the solution accuracy of the shell elements, different triangular mesh patterns and distorted element mesh are adopted in the benchmark problems. All shell finite elements considered pass the basic tests; namely, the isotropy, the patch, and the zero energy mode tests.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 추계학술대회 논문집
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• pp.830-831
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• 2009

• 한국전산구조공학회논문집
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• 제16권2호
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• pp.197-206
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• 2003

임의의 경계조건과 변두께를 갖는 축대칭 반구형 쉘과 반구형체의 진동수와 모우드형상을 결정하는 3차원적 해석법이 소개되었다. 수학적으로 2차원적인 전통적인 쉘이론과는 달리 본 연구의 해석법은 3차원 동적 탄성방정식을 사용하였다 자오선방향 (Φ), 법선방향(z), 원주방향(θ)으로의 변위성분인 μ/sub Φ/, μ/sub z/, μ/sub θ/는 시간에 대해서는 정현적으로, θ에 대해서는 주기적으로, 와 z 방향에 대해서는 대수다항식으로 표현될 수 있다. 축대칭 반구형 쉘의 변형률 에너지와 운동 에너지를 정식화하고, 리츠법으로 고유치문제를 계산하였다. 진동수의 최소화과정을 통해 엄밀해의 상위 경계치 진동수를 구하였으며, 이 때, 다항식의 차수를 증가시키면 진동수는 엄밀해에 수렴하게 된다. 자오선방향으로 선형적으로 꿩 두께가 변하는 반구형 쉘과 반구형체치 3차원적 진동수를 최초로 계산하였으며, 축방향으로 난 조그만 원추형 구멍이 진동수에 미치는 영향도 분석하였다. 상두께와 자유경계조건을 갖는 두꺼운 축대칭 반구형 쉘에 대한 3차원적 리츠해와 3차원적 유한요소법에 의한 진동수를 서로 비교하였다.

• 제어로봇시스템학회논문지
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• 제20권5호
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• pp.563-569
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• 2014

This article focuses on a hemispherical resonator gyro driven by the Coriolis effect. A hemispherical shell, called a resonator, is maintained in the resonance state by amplitude control and phase locking control. Parametric excitation has been used to control the amplitude. For rate measurement mode or FTR mode, nodal points have been kept to an amplitude of zero. Angular rate measurement has been demonstrated by rotating a resonator. Frequency mismatch between two stiffness principal axes is a major cause of low performance: vibrating pattern drift and reduced control effectiveness. This mismatch has been reduced significantly by the addition of small mass. A negative spring effect, which lowers resonance frequencies, has been verified experimentally.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2005년도 춘계학술대회 논문집
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• pp.702-705
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• 2005

Metallic sandwich plates with Inner dimpled shell subjected to 3-point bending have been analyzed and then optimized for minimum weight. Inner dimpled shells can be easily fabricated by press or roll with high quality precision and bonded with same material skin sheets by resistance welding or adhesive bonding process. Optimized shape of inner dimple is a hemispherical shell to minimize weight without failure, including face yielding, face buckling and inner dimple buckling. It is demonstrated that bending stiffness of sandwich plate is 2 or 3 times than solid plates with same strength

• Structural Engineering and Mechanics
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• 제55권2호
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• pp.245-261
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• 2015

A three-dimensional (3-D) method of analysis is presented for determining the free vibration frequencies of eccentric hemi-spherical shells of revolution with variable thickness. Unlike conventional shell theories, which are mathematically two-dimensional (2-D), the present method is based upon the 3-D dynamic equations of elasticity. Displacement components $u_r$, $u_{\Theta}$, and $u_z$ in the radial, circumferential, and axial directions, respectively, are taken to be periodic in ${\theta}$ and in time, and algebraic polynomials in the r and z directions. Potential and kinetic energies of eccentric hemi-spherical shells with variable thickness are formulated, and the Ritz method is used to solve the eigenvalue problem, thus yielding upper bound values of the frequencies by minimizing the frequencies. As the degree of the polynomials is increased, frequencies converge to the exact values. Convergence to three or four-digit exactitude is demonstrated for the first five frequencies of the shells. Numerical results are presented for a variety of eccentric hemi-spherical shells with variable thickness.

• 한국세라믹학회지
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• 제44권7호
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• pp.368-374
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• 2007

CVD-SiC coating has been introduced as a protective layer in TRISO nuclear fuel particle of high temperature gas cooled reactor (HTGR) due to its excellent mechanical stability at high temperature. In order to prevent the failure of the TRISO particles, it is important to evaluate the fracture strength of the SiC coating layer. It is needed to develop a new simple characterization technique to evaluate the mechanical properties of the coating layer as a pre-irradiation step. In present work, direct strength measurement method with the specimen of hem i-spherical shell configuration was suggested. The indentation experiment on a hemisphere shell with a plate indenter was conducted. The fracture strength of the coating layer is related with the critical load for radial cracking of the shell. The finite element analysis was used to drive the semi-empirical equation for the strength measurement. The SiC hemispherical shells were successfully recovered from the section-grinding of TRISO coated particle and successive heat treatment in air. The strength of CVD-SiC coating layer was evaluated from the experimentally measured critical load during the indentation on SiC hemisphere shell. Weibull diagram of fracture strength was also constructed. This study suggested a new strength equation and experimental method to measure the fracture strength of CVD-SiC coating of TRISO coated fuel particles.

• 한국정밀공학회지
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• 제23권8호
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• pp.127-136
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• 2006

Metallic sandwich plates with inner dimpled shell subject to 3-point bending have been analyzed and then optimized for minimum weight. Inner dimpled shells can be easily fabricated by press or roll with high precision and bonded with same material skin sheets by resistance welding or adhesive bonding. Metallic sandwich plates with inner dimpled shell structure can be optimally designed for minimum weight subject to prescribed combination of bending and transverse shear loads. Fundamental findings for lightweight design are presented through constrained optimization. Failure responses of sandwich plates are predicted and formulated with an assumption of narrow sandwich beam theory. Failure is attributed to four kinds of mechanisms: face yielding, face buckling, dimple buckling and dimple collapse. Optimized shape of inner dimpled shell structure is a hemispherical shell to minimize weight without failure. It is demonstrated that bending stiffness of sandwich plate is 2 or 3 times larger than solid plates with the same strength. Failure mode boundaries and iso-strength lines dependent upon the geometry and yield strain of the material are plotted with respect to geometric parameters on the failure map. Because optimal parameters of maximum strength for given material weight can be selected from the map, analytic solutions for maximum strength are expressed as a function of only material property and proposed strength. These optimal parameters match well with numerical optimal parameters.